I did a little research and the pieces of this plan are becoming clear. Virtio appears to be a totally reasonable platform abstraction API, and KVM will do the job as a hypervisor. I’ll set up an x86_64-elf gcc crosscompiler and use newlib as the C library. Each executable will have its own disk image, and exec will function by spawning a new VM and booting it with the target executable.

The missing piece, so far as I can tell, is a proxy representation of the hypervisor’s management interface which can be provided to a guest OS, so that our VMs can virtualize themselves – and pass on a proxy for their own hypervisor proxy, so that subprocesses can virtualize themselves in turn, recursively. This would enable the construction of a guest-OS shell managing an array of processes which are themselves independent guest-OS machines. Current thought: define the ‘virsh’ terminal interface as a serial protocol, then write a linux-side launcher process that creates a pipe-based virtual serial device and hands it off when starting up the first guest process.

With the launcher and the multitasking shell in place, a toolchain targeting this baremetal environment, and an array of virtio device drivers in the form of static libs you can link in, the platform would be ready to go.